1. Field of the Invention
The invention relates to the preparation of molded polyurethane parts having a lightfast polyurethane skin. The skin is formed by applying a reaction mixture comprising an organic polyisocyanate, a polyol, a chain extender, an aliphatic diamine, and a catalyst, by a one-shot process in the absence of a solvent, to the inside wall of a molding tool.
2. Description of the Prior Art
The preparation of cross-linked plastics by the isocyanate-polyaddition process is known in the art. According to German Application No. 1,196,864 (U.S. Pat. No. 3,099,516), hydroxyl group containing compounds and polyisocyantes are foamed in molds in the presence of blowing agents and catalysts to prepare cross-linked plastics. Flexible and rigid polyurethane elastomers, as well as modifications between these two extremes, can be produced according to this process with suitable selection of the hydroxyl group containing compounds and organic polyisocyanates as well as by the additional use of chain extenders such as glycols or diamines.
In order to prepare polyurethane elastomers, isocyanate group containing prepolymers are initially produced from the hydroxyl group containing compounds and polyisocyanates which are then reacted in a second stage with chain extenders to result in high molecular weight elastomers. See for example German Pat. No. 831,604 (U.S. Pat. No. 2,778,810).
If diamines are used as chain extenders, variations in processing must be made. According to German Application No. 1,149,523 (U.S. Pat. No. 3,105,062) crystalline aromatic diprimary diamines are incorporated in the liquid isocyanate group-containing prepolymers, in a quantity which is insufficient for saturating the isocyanate groups, at a temperature below the melting point of the diamines. The masses are subsequently cured by heating. In accordance with German Pat. No. 1,240,654 (U.S. Pat. No. 3,428,610), isocyanate group-containing prepolymers are reacted at room temperature or modestly increased temperatures with liquid or dissolved aromatic diamines which have at least one linear alkyl substituent in the ortho position to the first amino group and two linear alkyl substituents with one to three carbon atoms in the ortho position to the second amino group.
In order to produce molded polyurethane parts with a flexible surface skin, solutions of isocyanate group containing prepolymers and diamine solutions are mixed and are sprayed onto the substrate, for instance, the inside wall of a molding tool. After evaporation of the solvent, a polyurethane-polyurea coating is obtained. See for example German Published Application No. 1,816,554 (U.S. Pat. No. 3,576,930). Lightfast molded polyurethane parts can also be produced according to the following processes:
(1) Integral foam formulations based on aliphatic polyisocyanates may be foamed, for instance, according to processes analogous to German Published Application No. 2,710,901 (U.S. Pat. No. 4,025,466). The problem with this method is that expensive aliphatic polyisocyanates are consumed in large quantities and that a uniformly unfoamed surface which is free of color schlieren is rarely achieved. PA1 (2) Completed molded foam parts may be coated with polyurethane coatings. This mode of operation generally results in adhesion problems between the surface layer and the foam core since the completed foam core always contains residues of the mold release agent at the surface. An added problem with thicker layers is that an undistorted shaping of the surface of the foam core cannot be guaranteed. This is a problem particularly in those cases where the surface structure of the foam core, for instance, a leatherlike graining, should remain visible. PA1 (3) "In-mold coating", that is by coating the inside wall of the molding tool with lightfast solvent containing polyurethane systems. The problems with this mode of operation are the expensive drying stages which may cause long cycle times and the considerable stress on the environment. If solvent-free polyurethane formulations are used, lightfast surfaces could not be sprayed according to the two-component process because of low reactivity of aliphatic polyisocyanates used in such systems. The solvent-free formulations remain very thin for a long period of time and run down the vertical walls of the mold and collect in the horizontal parts of the molds. It, therefore, proved impossible to produce uniform coatings with constant layer thickness.